Nowadays, surface-treated steel sheets manufactured by giving corrosion resistance to raw material steel sheets are being used in the fields of, for example, automobile, domestic electric appliance, and building material industries. Among the surface-treated steel sheets described above, hot dip galvanized steel sheets, which are excellent in terms of corrosion resistance, are being favorably used. In particular, car makers in Europe and the United States are thinking of increasing the corrosion resistance of raw material steel sheets by using hot dip galvanized steel sheets whose coating thickness can be easily increased. In addition, a large demand for automotive steel sheets is expected in the East Asia region, which is showing significant economic growth.
In addition, in automotive steel sheets, which are strictly required to have excellent workability, it is not possible to achieve sufficient durability for products unless coating adhesiveness after press working has been performed and after-painting corrosion resistance after press working has been performed are excellent.
Also, high-strength steel sheets, which are used as, in particular, strength members, are required to have sufficient press workability against intense working and satisfactory corrosion resistance in a portion which has been subjected to press working. Therefore, the coating adhesiveness in the worked portion mentioned above is very important.
Japanese Unexamined Patent Application Publication No. 2004-315965 discloses a method of manufacturing a hot dip galvanized steel sheet excellent in terms of slidability when press working is performed in which the amount of Al in a hot dip galvanizing layer and the amount of Al at the interface between the coated layer and the steel sheet are specified. However, in JP '965, sufficient consideration is not given to the durability of products related to, for example, coating adhesiveness in a worked portion after press working has been performed and corrosion resistance after press working has been performed.
Currently, as described above, it is difficult to obtain steel sheets having durability by giving the steel sheet excellent coating adhesiveness after press working has been performed and after-painting corrosion resistance after press working has been performed.
In addition, since hot dip galvanized steel sheets are used in the fields of, for example, automobile, domestic electric appliance, and building material industries, hot dip galvanized steel sheets are also required to have excellent after-painting beautiful appearance and spot weldability.
It could therefore be helpful to provide a hot dip galvanized steel sheet excellent in terms of coating adhesiveness after press working has been performed, spot weldability, after-painting corrosion resistance after press working has been performed, and after-painting beautiful appearance.